The primary aim of this project is to develop, implement and evaluate an intervention that demonstrates the efficacy of a relatively inexpensive, long-term, Internet-delivered program for improving physical activity, fitness, and nutrition behaviors among sedentary adults with high-normal, overweight, or moderately obese body mass indices. This intervention will feature an automated "mastery-model" lifestyle coach as a means to achieve the disease reduction benefits: 1) Meet the Surgeon General's physical activity recommendations of adding 30 minutes of moderate activity most days of the week or the step-count equivalent of increasing an individual's daily steps by 3000 steps/day. 2) Increase cardiorespiratory fitness by 1-1.5 METs or the walk-time equivalent of decreasing an individual's one-mile walk time by 60-90 seconds). 3) Meet the Surgeon General's nutrition recommendations of eating 30 percent or less total calories from fat, at least five servings of fruits and vegetables, and at least 25-30 grams of dietary fiber per day. The project will enroll sedentary adults with BMI levels of 23-35 to yield 112 participants at the 24-month follow-up in each of two study groups. The project will use a control-randomized experimental design comparing the mastery-model intervention to a standard-care Internet intervention. Measures will include objective measures of physical activity and fitness, food frequency measures of dietary intake, and measures reflecting the SCT model of behavior change. An intent-to-treat, repeated measures analysis of covariance model will test for treatment effects. Structural equation modeling will be used to test the social cognitive model underlying the intervention. The project will demonstrate that the Internet can be used to deliver an individualized, tailored intervention that provides automated mastery-model or lifestyle coaching following a theory-based protocol and can produce changes in physical activity, fitness, and nutrition comparable to those reported by other long-term community-based interventions.